Among the various video display systems available in the art, an optical projection system is known to be capable of providing a high quality display in a large scale. In such an optical projection system, light from a lamp is uniformly illuminated onto an array of, e.g., M.times.N, actuated mirrors such that each of the mirrors is coupled with each of the actuators. The actuators may be made of an electrodisplacive material such as a piezoelectric or an electrostrictive material which deforms in response to an electric field applied thereto.
The reflected light beam from each of the mirrors is incident upon an aperture of a baffle. By applying an electrical signal to each of the actuators, the relative position of each of the mirrors to the incident light beam is altered, thereby causing a deviation in the optical path of the reflected beam from each of the mirrors. As the optical path of each of the reflected beams is varied, the amount of light reflected from each of the mirrors which passes through the aperture is changed, thereby modulating the intensity of the beam. The modulated beams through the aperture are transmitted onto a projection screen via an appropriate optical device such as a projection lens, to thereby display an image thereon.
In FIG. 1, there is shown a cross sectional view of a prior art piezoelectric mirror actuator 10 used in an M.times.N piezoelectric actuated mirror array as disclosed in U.S. Pat. No. 5,159,225, entitled "PIEZOELECTRIC ACTUATOR", comprising a substrate 14, a mirror 12, a pair of piezoelectric members 16a, 16b and a hinge 18.
Each of the piezoelectric members 16a, 16b has a first side surface 20a, 20b, a second side surface 22a, 22b, a top surface 24a, 24b and a bottom surface 26a, 26b. The bottom surface 26a, 26b is mounted to the substrate 14. In this embodiment, the first side surface 20a, 20b of each of the piezoelectric members is in a facing relationship to each other, and the second side surface 22a, 22b is opposite therefrom.
The polarization of the piezoelectric members 16a, 16b is selected so that when a voltage is applied between their respective first side suface 20a, 20b and second side surface 22a, 22b, each member 16a, 16b will shear in a direction determined by the polarity of the voltage. By placing the first side surface 20a, 20b of each member 16a, 16b in a facing relationship, the application of an identical voltage will cause the top surface 24a, 24b of each member 16a, 16b to shear toward each other for a first polarity or shear away from each other for a second, opposite polarity of the applied voltage. Other orientations and polarizations of the piezoelectric members 16a, 16b when combined with a proper voltage and polarity applied to each member 16a, 16b are also possible.
To apply such voltage, a first metallization 28 is deposited, by employing one of the conventional techniques, to interconnect the first side surface 20a, 20b of each member 16a, 16b. Similarly, a second metallization 30 is applied to each second side surface 22a, 22b. The second metallization may be coupled to a common, ground potential. Application of the voltage to the first metallization 28 will therefore cause each piezoelectric member 16a, 16b to shear.
The hinge 18 includes a pair of mounting members 32a, 32b, a pair of translational members 34a, 34b, a rotating member 36 and a driven member 38. Each of the mounting members 32 is mounted on the top surface 24a, 24b of a respective one of the piezoelectric members 16a, 16b. The rotating member 36 is disposed between each mounting member 32a, 32b and attached thereto via each translational members 34a, 34b. More particularly, the first translational member 34a is mounted at its distal end 40a to an upper portion 42 of the rotating member 36 and the second translational member 34b is mounted at its distal end 40b to a lower portion 44 of the rotating member 36. The driven member 38 is attached to the rotating member 36 and extends radially outward from the axis of rotation.
A distal end 46 of the driven member may include a planar platfrom 48 to which the mirror 12 is mounted. The application of a voltage will cause shearing as described hereinabove to tilt the plane of the mirror 12 in response to the rotation of the rotating member 36.
An array of such actuators 10 may be constructed to be individually addressable. An opening 50 is formed, through the substrate 14, located between the pair of piezoelectric members 16a, 16b in the array of such pairs. A metallization 52 fills the hole and is in electrical contact with the first metallization 28. An addressable driver 54 mounted to the lower surface of the substrate 14 applies a voltage to the metallization 52. The voltage at each pair in the array may be developed in accodance with pixel intensity in a light modulator projection apparatus. The second metallization 30 at each second face 24a, 24b may be interconnected in common on the top surface of the substrate.
There are various problems associated with the prior art piezoelectric mirror actuator. They are: (1) since the piezoelectric material, e.g., lead zirconium titanate(PZT), exhibits a hysteresis, it is rather difficult to obtain a consistent and accurate mechanical response for a given electric signal; (2) the piezoelectric material must be poled by using a very cumbersome process; and (3) since the piezoelectric mirror actuator is manufactured using mechanical means, e.g., sawing, it is difficult to obtain the desired yield, and, furthermore, there is a limit to the down sizing thereof.